The invention relates to the treatment of nitrogen-laden natural gas. More particularly, the invention relates to the removal of nitrogen from such natural gas by means of gas-separation membranes.
Fourteen percent of known U.S. natural gas reserves contain more than 4% nitrogen. Many of these reserves cannot be exploited because no economical technology for removing the nitrogen exists.
Cryogenic distillation is the only process that has been used to date on any scale to remove nitrogen from natural gas. The gas streams that have been treated by cryogenic distillation, for example streams from enhanced oil recovery, have large flow rates and high nitrogen concentration, such as more than 10 vol %. Cryogenic plants can be cost-effective in these applications because all the separated products have value. The propane, butane and heavier hydrocarbons can be recovered as natural gas liquids (NGL), the methane/ethane stream can be delivered to the gas pipeline and the nitrogen can be reinjected into the formation.
Cryogenic plants are not used more widely because they are costly and complicated. A particular complication is the need for significant pretreatment to remove water vapor, carbon dioxide and C3+ hydrocarbons and aromatics to avoid freezing of these components in the cryogenic section of the plant, which typically operates at temperatures down to xe2x88x92150xc2x0 C. The degree of pretreatment is often far more elaborate and the demands placed upon it are far more stringent than would be required to render the gas acceptable in the pipeline absent the excess nitrogen content. For example, pipeline specification for water vapor is generally about 120 ppm; to be fit to enter a cryogenic plant, the gas must contain no more than 1-2 ppm of water vapor at most. Similarly, 2% carbon dioxide content may pass muster in the pipeline, whereas carbon dioxide must be present at levels no higher than about 100 ppm for cryogenic separation. For streams of flow rates less than about 50-100 MMscfd, therefore, cryogenic technology is simply too expensive and impractical for use.
Other processes that have been considered for performing this separation include pressure swing adsorption and lean oil absorption; none is believed to be in regular industrial use.
Gas separation by means of membrane is known. For example, numerous patents describe membranes and membrane processes for separating oxygen or nitrogen from air, hydrogen from various gas streams and carbon dioxide from natural gas. Such processes are in industrial use, using glassy polymeric membranes. Rubbery polymeric membranes are used to separate organic components from air or other gas mixtures.
A report by SRI to the U.S. Department of Energy (xe2x80x9cEnergy Minimization of Separation Processes using Conventional Membrane/Hybrid Systemsxe2x80x9d, D. E. Gottschlich et al., final report under contract number DE 91-004710, 1990) suggests that separation of nitrogen from methane might be achieved by a hybrid membrane/pressure swing adsorption system. The report shows and considers several designs, assuming that a hypothetical nitrogen-selective membrane, with a selectivity for nitrogen over methane of 5 and a trans membrane methane flux of 1xc3x9710xe2x88x926 cm3(STP)/cm2xc2x7sxc2x7cmHg, were to become available, which to date it has not.
In fact, both glassy and rubbery membranes have poor selectivities for nitrogen over methane or methane over nitrogen. Table 1 lists some representative values.
These separation properties are not good enough to make methane/nitrogen separation by membrane practical, either by preferentially permeating the nitrogen or by preferentially permeating the methane.
The problem of separating gas mixtures containing methane and nitrogen into a methane-rich stream and a nitrogen-rich stream is, therefore, a very difficult one, owing to the low selectivity of essentially all membrane materials to these gases. However, it was discovered a few years ago that operating silicone rubber membranes at low temperatures can increase the methane/nitrogen selectivity to as high as 5, 6 or above. U.S. Pat. Nos. 5,669,958 and 5,647,227 make use of this discovery and disclose low-temperature methane/nitrogen separation processes using silicone rubber or similar membranes to preferentially permeate methane and reject nitrogen. However, such a selectivity is obtained only at very low temperatures, typically xe2x88x9260xc2x0 C., for example. Temperatures this low generally cannot be reached by relying on the Joule-Thomson effect to cool the membrane permeate and residue streams, but necessitate additional chilling by means of external refrigeration. While such processes may be workable in industrial facilities with ready access to refrigeration plants, they are impractical in many gas fields, where equipment must be simple, robust and able to function for long periods without operator attention.
Another problem of very low temperature operation is that, even though the membranes themselves may withstand the presence of liquid water and hydrocarbons, considerable pretreatment is often necessary to avoid damage to ancillary equipment by condensed liquids. Streams must also be dried to a very low water content to prevent the formation of methane or other hydrocarbon hydrates that can clog the system.
Yet another problem of very low temperature operation is that equipment components must be made from comparatively expensive stainless steel or other special steels, rather than lower cost carbon steels.
Further concerns that hamper membrane process design for methane/nitrogen separation are that vacuum pumps generally must not be used anywhere in the system as they may permit air to leak into lines carrying hydrocarbon mixtures, representing an unacceptable safety hazard. Indeed, for safety, reliability and cost-containment, the number of pieces of rotating or moving equipment of any kind should be kept to a minimum.
In view of these multiple difficulties, there remains an unsatisfied need for economical means of exploiting nitrogen-rich natural gas reserves, especially those contained in gas fields with smaller flow rates.
The invention is a process for treating natural gas or other methane-rich gas to remove excess nitrogen, thereby producing one, two or three product streams of value. The invention relies on membrane separation using methane-selective membranes, but does not require the membranes to be operated at very low temperatures. We have found that, by using a two-step membrane system design, and optionally controlling the operating parameters for the membrane steps within certain ranges, the capital and operating costs of the process can be kept within economically acceptable limits.
Two basic process configurations can be used, depending on the pressure of the raw gas to be treated by the process. If the raw gas is already at comparatively high pressure, the invention includes the following steps:
(a) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, the first membrane being more permeable to methane than to nitrogen;
(b) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, the second membrane being more permeable to methane than to nitrogen, the second membrane unit being connected in series with the first membrane unit such that gas leaving the first feed side can enter the second membrane unit on the second feed side;
(c) passing a gas stream, comprising methane and at least about 4% nitrogen, at a first pressure, and at a first temperature higher than about xe2x88x9240xc2x0 C., into the first membrane unit at a first inlet of the first feed side;
(d) withdrawing from a first outlet of the first feed side a first residue stream enriched in nitrogen compared with the gas stream;
(e) withdrawing from the first permeate side as a first product stream, at a second pressure lower than the first pressure, a first permeate stream depleted in nitrogen compared with the gas stream;
(f) passing the first residue stream, at a second temperature, into the second membrane unit at a second inlet of the second feed side;
(g) withdrawing from a second outlet of the second feed side a second residue stream enriched in nitrogen compared with the first residue stream;
(h) withdrawing from the second permeate side, at a third pressure lower than the first pressure, a second permeate stream depleted in nitrogen compared with the first residue stream.
If the raw gas is at comparatively low pressure, the invention includes the following steps:
(a) compressing a gas stream comprising methane and at least about 4% nitrogen to a first pressure in the range 400-1,500 psia to form a compressed gas stream;
(b) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, the first membrane being more permeable to methane than to nitrogen;
(c) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, the second membrane being more permeable to methane than to nitrogen, the second membrane unit being connected in series with the first membrane unit such that gas leaving the first feed side can enter the second membrane unit on the second feed side;
(d) introducing the compressed gas stream at a first temperature higher than about xe2x88x9240xc2x0 C. into the first membrane unit at a first inlet of the first feed side, and passing the compressed gas stream across the first feed side;
(e) withdrawing from a first outlet of the first feed side a first residue stream enriched in nitrogen compared with the gas stream;
(f) withdrawing from the first permeate side as a first product stream, at a second pressure lower than the first pressure and above about 25 psia, a first permeate stream depleted in nitrogen compared with the gas stream;
(g) introducing the first residue stream into the second membrane unit at a second inlet of the second feed side, and passing the first residue stream across the second feed side;
(h) withdrawing from a second outlet of the second feed side a second residue stream enriched in nitrogen compared with the first residue stream;
(i) withdrawing from the second permeate side, at a third pressure lower than the first pressure and above about 25 psia, a second permeate stream depleted in nitrogen compared with the first residue stream.
Preferred embodiments of the invention, depending on the feed gas composition, include either recirculating the second permeate stream to the front of the process to increase methane recovery, or withdrawing the second permeate stream as a second product stream, specifically for use as fuel to run the compressor or other field equipment.
The second residue stream is usually flared, used as fuel or reinjected. As a preferred alternative, it is possible to include a third, smaller membrane separation step to treat the second residue stream by fractionating it into a comparatively methane-rich permeate stream, which may optionally be used as engine fuel to drive the compressor, and a comparatively nitrogen-rich residue stream, which may be flared or reinjected, for example.
By adopting one of these preferred embodiments, the fuel to run any compressor needed for the process can be generated as a discrete product stream by the process itself. This is very beneficial as gas-fired compressors can operate in remote locations where an electrical power supply is unavailable.
The process of the invention offers a number of additional features and advantages. Most importantly, it enables natural gas containing relatively large amounts of nitrogen, such as 10%, 20% or higher, to be brought close to or within pipeline specification of no more than 4% nitrogen. Furthermore, for small gas streams or remote gas fields, these results can be achieved more simply, reliably and cheaply than could be done with prior art technology.
Also, unlike the prior art membrane processes disclosed in U.S. Pat. Nos. 5,669,958 and 5,647,227, it is not necessary to operate the membrane separation steps under conditions of such low temperature as to yield a methane/nitrogen selectivity of at least 5. The two-step membrane process configuration, optionally combined with operation in an optimum pressure range, provides adequate performance, in terms of low product nitrogen content combined with good methane recovery, even when the membrane selectivity is as low as 2, 3 or 4, for example. Sufficient cooling to produce adequate selectivity can, therefore, be provided in most cases simply by taking advantage of the cooling by Joule-Thomson effect of both permeate and residue streams that takes place in membrane separation processes.
This effect is discussed at length in, for example, U.S. Pat. No. 5,762,685. The feed and permeate sides of a membrane are separated only by the very thin polymer membrane layer and are in good thermal contact. Thus, although it is expansion to the permeate side that produces the cooling, membrane separation of a gas stream containing organic components typically results in the residue stream, as well as the permeate, being significantly colder than the gas that was fed to the membrane. In experimental tests, we have found in some cases that the residue and permeate streams are at about the same temperature; in other cases we have found that the residue stream is the colder. Either the residue or the permeate, or both, can, therefore, be used to cool the incoming gas.
Such cooling can be accomplished by heat exchange between the membrane feed, residue and permeate streams, and optionally by expanding the membrane residue stream before such heat exchange, without the need for any external refrigeration source. In general, the process can be operated at temperatures above xe2x88x9240xc2x0 C., and often much higher, such as above xe2x88x9230xc2x0 C., above xe2x88x9225xc2x0 C., above xe2x88x9210xc2x0 C. or even around 0xc2x0 C. or above. The ability to function at these comparatively high temperatures and without external cooling in many instances is a particular advantage of the present invention, as it greatly simplifies the process compared with prior art technologies.
The most important product of the process is the methane-rich, nitrogen-depleted permeate stream, which must frequently meet a specification of no more than about 4% nitrogen. Since a controlled permeate composition is a key target of the process, it is conventional in the membrane separation arts to achieve this composition, if it cannot be reached in a single membrane-separation stage, by using a two- or multi-stage configuration, in which the inadequately enriched permeate from the first stage is passed, often after recompression, as feed to a second membrane separation stage, and so on, until the desired composition has been achieved.
In contrast, the process of the invention relies on two or more membranes steps, rather than two or more membrane stages, to reach a permeate composition of desired enrichment of methane and depletion of nitrogen. For this reason, it is generally possible to use only one compression step in the process. This is very advantageous, as it enhances reliability and acceptability in the field, and can result in cost and energy savings.
Furthermore, very high pressures are not needed for good performance. Rather, we have discovered that the capital costs of the equipment and the compressor horse power required to perform the process both tend to pass through minimum values when the feed gas pressure to the first membrane separation step is between about 400 psia and 1,500 psia.
If desired, the process can be operated so as to keep the average temperatures of the membrane separation units and incoming and outgoing streams above about xe2x88x9225xc2x0 C. In this case, metal components of the equipment can be made from carbon steel rather than stainless steel, with considerable cost savings.
A final advantage is that the membranes can operate in the presence of water, carbon dioxide and C3+hydrocarbons, all of which are almost always present in natural gas to some extent. These components have no adverse effects on the membranes, but simply pass into the permeate stream along with the methane, even if the membrane separation is performed at conditions close to the water or hydrocarbon dewpoints. These capabilities are in sharp contrast to cryogenic methane/nitrogen separation, where the presence of even low ppm levels of these contaminants can be problematic. Also, since the process of the present invention operates at relatively high temperatures, such as above xe2x88x9240xc2x0 C., above xe2x88x9225xc2x0 C., above xe2x88x9210xc2x0 C. or even around 10xc2x0 C. or above, the formation of hydrates or liquified hydrocarbons is much less likely than in previous processes. Thus, unlike cryogenic separation and previous membrane separation processes, the process of the invention can be carried out, if desired, with little or no pretreatment of the incoming raw natural gas.
In another aspect, the invention is a process for producing one, two or three product streams of different nitrogen content, all of which may have value, from a nitrogen-contaminated natural gas stream that previously would have been of little or no value. In this aspect, the process includes two, three or more membrane separation steps connected in series as described above, so that the residue stream from the first step flows as feed to the second step, and so on.
The first membrane separation step produces the first product stream of value, a low-nitrogen, high-methane permeate stream. This nitrogen-depleted, hydrocarbon-enriched product stream typically contains no more than about 6%, more preferably no more than about 4%, nitrogen. Typically, the first product stream also contains at least about 70%, more preferably at least about 80%, of the methane content (or, where significant amounts of C2+ hydrocarbons are also present, the total hydrocarbon content) of the feed stream.
The last membrane separation step produces two streams, a last permeate stream and a last residue stream, one or both of which may be product streams of value. The second stream of value is the permeate stream from the last step. This stream typically, and preferably, has no more than about 40% nitrogen, more preferably no more than about 35% or 30% nitrogen. Typically, this stream also has at least about 50% methane, plus small amounts of ethane and C3+ hydrocarbons. Gas of this composition generally has a Btu value of at least about 700 Btu/scf, high enough to be a good source of compressor fuel gas.
The residue stream from the last membrane separation step is the third stream of value. This stream typically contains at least about 40% nitrogen, and often at least 50% nitrogen, 60% nitrogen or more. This stream also typically has a methane content no higher than about 50%, and preferably no higher than about 45% methane or 40% methane. This nitrogen-rich stream is still at pressure, and has value as an injectant gas into the formation producing the raw gas.
The invention is particularly useful for treating gas streams that arise as a result of nitrogen injection processes. Traditional oil-production techniques recover as little as 25-35% of the oil in a typical field. Recovery is improved by injecting carbon dioxide or nitrogen into the reservoir at the periphery. The gas dissolves in the remaining oil and lowers its viscosity, enabling it to be pushed more readily to the extraction wells. High-pressure nitrogen is also injected into gas fields to drive the gas to the wells, as well as to recover methane from coal bed methane reservoirs. The overall economics of such processes are dependent on the costs of the nitrogen injectant, which often has to be supplied from a cryogenic plant on site or a similarly costly source. A cost-effective process able to recover nitrogen at a composition suitable for reinjection makes these types of processes more efficient and attractive. The invention, particularly in its last aspect, provides such processes.
It is an object of the invention to provide a process for removing excess nitrogen from methane/nitrogen gas mixtures.
It is an object of the invention to provide a method for removing excess nitrogen from natural gas without cooling the gas to very low temperatures, such as below xe2x88x9240xc2x0 C.
It is an object of the invention to provide a process for producing one, two or three streams of value from nitrogen-contaminated methane streams.
It is an object of the invention to provide a simple, reliable and cost-effective method for processing nitrogen-contaminated natural gas from small or remote fields.
It is an object of the invention to provide membrane-based processes that use only one compression step for removing excess nitrogen from natural gas.
Other objects and advantages will be apparent from the description of the invention to those skilled in the gas separation arts.